MOCVD is a method for depositing dopants or thin metal or metal compound films on a silicon or other substrate. (In the present disclosure "metal" includes all of the elements of Groups 2B, 2A, 3A, 4A, 5A, and 6A of the Periodic Table except carbon, nitrogen, oxygen, and sulfur.) The deposited films can be sources of doping impurities which are driven into the substrate, or the films themselves can have different electrical or optical properties than the substrate. These films are used in laser diodes, solar cells, photocathodes, field effect transistors and other discrete devices, in fiber optic communications, microwave communications, digital audio disc systems, and other advanced semiconductor, optical, and optoelectronic technologies. The properties of the film depend on the deposition conditions and the chemical identity of the deposited film.
A special advantage of MOCVD is that organometallic compounds can be found which have much higher vapor pressures at moderate temperatures than the corresponding metals, and which decompose to release the corresponding metals or form compounds thereof at the 200 to 800 degrees Celsius deposition temperatures which should not be exceeded during fabrication.
Typical apparatus currently in use for MOCVD comprises a bubbler which contains a supply of the organometallic compound chosen for a particular process, a reactor or deposition chamber which contains the substrate on which a film is to be deposited, a source of a carrier gas which is inert to the organometallic compound in the bubbler and either inert or reactive to the compound in the deposition chamber, and optionally sources of other reactants or dopants supplied to the reaction chamber. The bubbler and contents are maintained at a constant and relatively low temperature which typically is above the melting point of the organometallic compound but far below its decomposition temperature. The deposition chamber is typically maintained at a much higher temperature, such as about 200 to 800 degrees Celsius, particularly about 600 to 750 degrees Celsius, at which the organometallic compound readily decomposes to release its constituent metal. To operate the MOCVD apparatus, the carrier gas is introduced into the bubbler under the surface of the organometallic compound. Rising bubbles of the carrier gas provide a large, constant contact surface and thus uniformly vaporize the organometallic compound. The carrier gas and vapor collected in the headspace of the bubbler are continuously directed to the deposition chamber.
While it is possible to vaporize solid sources of arsenic or phosphorus in a bubbler or furnace (see Bhat, cited later), this way of providing arsenic or phosphorus has several disadvantages. First, when a III-V compound such as gallium arsenide is to be deposited the Group III element (here, gallium) is conventionally supplied from an organometallic compound such as trimethylgallium. The source of the Group V element should include hydride substituents so that monatomic hydrogen will be formed when the hydride decomposes in the deposition chamber. The monatomic hydrogen thus formed will react with the organic radicals (methyl radicals, in the case of trimethylgallium) formed by decomposition of the Group V source in the deposition chamber to form gaseous waste (here, methane gas), allowing the organic constituents to be removed from the site of deposition. For this reason, in prior practice a large excess of Group V hydride (here, arsine) has been supplied to ensure thorough removal of organic constituents. Elemental arsenic supplied to the deposition chamber would include no hydride substituents, and thus the resulting film would be contaminated with carbon from the Group III source.
Second, it is difficult to control the rate of vaporization of such solid sources because the surface area of a solid exposed to the carrier gas changes as vaporization proceeds. In contrast, a liquid contained in a bubbler with substantially vertical walls presents the same surface area to the carrier gas so long as the flow and bubble size of the carrier gas remains steady. Also, gases (defined here as materials having a vapor pressure which exceeds the pressure within the bubbler at convenient bubbler temperatures) are not preferred for MOCVD because gases cannot be evaporated at a uniform rate in a bubbler. For example, arsine and phosphine have been supplied as gases in pressurized cylinders and metered directly into the deposition chamber.
Organometallic compounds for MOCVD desirably are liquids at bubbler pressure and at a temperature between about -20.degree. C. and about 40.degree. C. Such compounds also should have a vapor pressure of at least about 1.0 torrs at the bubbler temperature, boil and decompose at temperatures substantially exceeding the bubbler temperature, and decompose readily at the temperature encountered in the deposition chamber.
Another problem facing practitioners of MOCVD is that arsine and phosphine, commonly employed as sources of arsenic- and phosphorus-containing deposition products, are highly toxic. They have been the subject of proposed and existing restrictive legislation. The triorganometallic compounds previously proposed to replace them, such as trimethylarsine, are far less toxic, but leave residual carbon decomposition products in the deposited films. (See Bhat, "OMCVD Growth of GaAs and AlGaAs Using a Solid as a Source", Journal of Electronic Materials, Vol. 14, No. 4, 1985, pp.433-449. Kuech, et al. "Reduction of Background Doping in Metal-Organic Vapor Phase Epitaxy of GaAs Using Triethyl Gallium at Low Reactor Pressures," Appl. Phys. Letters I, Oct. 15, 1985, not believed to be prior art, also may have relevance because it discloses that gallium arsenide films made with trimethylgallium and arsine have more carbon residue than films made with triethylgallium.) Also, some triorganometallic compounds of arsenic and phosphorus may not have vapor pressures suitable for practicing MOCVD at a convenient temperature. Thus, new compounds which are effective replacements for arsine and phosphine, but less toxic, would be highly desirable.
The known organometallic compounds of elements in Groups 2B, 2A, 3A, 5A, or 6A of the Periodic Table, particularly bismuth, selenium, tellurium, beryllium, magnesium, or elements of Groups 2B or 3A of the Periodic Table, are relatively few in number. For example, the following compounds of these elements are all those lower alkyl, phenyl, alkyl-substituted phenyl, cyclopentadienyl, or alkyl-substituted cyclopentadienyl organometallic compounds listed in the CRC Handbook of Chemistry and Physics, 61st Edition, CRC Press, Inc., Boca Raton, Fla.:
ZINC PA0 CADMIUM PA0 MERCURY PA0 BERYLLIUM PA0 MAGNESIUM PA0 BORON PA0 ALUMINUM PA0 GALLIUM PA0 INDIUM PA0 THALLIUM PA0 PHOSPHORUS PA0 ARSENIC PA0 ANTIMONY PA0 BISMUTH PA0 SELENIUM PA0 TELLURIUM
Di-n-butylzinc PA1 Diethylzinc PA1 Dimethylzinc PA1 Diphenylzinc PA1 Di-n-propylzinc PA1 Di-o-tolylzinc PA1 Dibutylcadmium PA1 Diethylcadmium PA1 Diisobutylcadmium PA1 Dimethylcadmium PA1 Dipropylcadmium PA1 Dibenzylmercury PA1 Di-n-butylmercury PA1 Diethylmercury PA1 Diisobutylmercury PA1 Diisopropylmercury PA1 Dimethylmercury PA1 Diphenylmercury PA1 Dipropylmercury PA1 Di-o-tolylmercury PA1 Di-m-tolylmercury PA1 Di-p-tolylmercury PA1 Di-n-butylberyllium PA1 Diethylberyllium PA1 Dimethylberyllium PA1 Dipropylberyllium PA1 Dimethylmagnesium PA1 Diphenylmagnesium PA1 Tribenzylboron PA1 Tri-n-butylboron PA1 Tri-t-butylboron PA1 Triethylboron PA1 Triisobutylboron PA1 Trimethylboron PA1 Triphenylboron PA1 Tri-n-propylboron PA1 Tri-sec-butylboron PA1 Tri-p-tolylboron PA1 Tri-p-xylylboron PA1 Diisobutylaluminum hydride PA1 Triethylaluminum PA1 Triisobutylaluminum PA1 Trimethylaluminum PA1 Triphenylaluminum PA1 Triethylgallium PA1 Trimethylgallium PA1 Trimethylindium PA1 Triethylindium PA1 Triethylthallium PA1 Trimethylthallium PA1 Trimethylphosphine PA1 Triethylphosphine PA1 Tripropylphosphine PA1 Tributylphosphine PA1 Triphenylphosphine PA1 Dimethylarsine PA1 Methylarsine PA1 Phenylarsine PA1 Tribenzylarsine PA1 Trimethylarsine PA1 Triphenylarsine PA1 Pentamethylantimony PA1 Phenyldimethylantimony PA1 Tributylstibene PA1 Triethylantimony PA1 Trimethylantimony PA1 Triphenylantimony PA1 Methylbismuthine PA1 Trimethylbismuthine PA1 Triethylbismuthine PA1 Triphenylbismuthine PA1 Tri-n-propylbismuth PA1 Diethylselenide PA1 Dimethyltelluride PA1 Diethyltelluride
Some additional compounds disclosed in the prior art include dicyclohexylphosphine (U.S. Pat. No. 3,547,881); various triorganophosphines (U.S. Ser. No. 691,598, filed Jan. 15, 1985); and the mono- and diorganic arsines and phosphines identified as prior art in Table II herein. See also Tzscach, et al., "Zur Synthese der Dialkylamine Dialkylarsine. Sowie der Dialkyl-arsine", Zeitschrift fur Anorganische und Allegemaine Chemie, Band 326, 1964 (pp. 280-287); and Horiguchu, et al., "Mass Spectrometric Study of Growth Reactions in Low Pressure CMVPE of GaAs by in situ Gas Sampling," presented at the 12th International Symposium on Gallium Arsenide and Related Compounds in Japan, 23-26 September, 1985 (this reference is not believed to be prior art).
Because there are few organometallic compounds of most of the listed elements, and particularly of aluminum, gallium, indium, selenium, tellurium, beryllium, and magnesium, there often will be no compound of a particular metal which is well suited to MOCVD. Furthermore, most of the previously listed compounds (with the exceptions of dimethylaluminum hydride, diethylaluminum hydride, diisobutylaluminum hydride, certain alkyl and dialkylarsines, phenylarsine, phenyldimethylantimony, and methylbismuthine) do not include more than one type of organic substituent on a given molecule. Particularly for Group 2B, 2A, and 3A elements of the Periodic Table it is difficult to select a useful candidate having the necessary properties for MOCVD.
Another factor complicates the selection of a workable organometallic compound for MOCVD: structurally related organometallic compounds often do not form homologous series. Many organometallic compounds characteristically exist in only one form, for example, as monomers, dimers, trimers, tetramers, or higher polymers. Structurally similar compounds often have different characteristic forms, and thus much different or inconsistent vapor pressures, melting points, and decomposition temperatures.
As a particular case in point, consider the two known compounds of indium--trimethylindium and triethylindium. Both of these compounds have been used to deposit indium containing films. (See: 1. Manasevit and Simpson, J. Electrochem. Soc., 118, C291 (1971); 120, 135 (1973). 2. Bass, J. Crystal Growth, 31, 172 (1975). 3. Duchemin, et al., Paper 13, 7th Intern. Symp. on GaAs and Related Compounds, Clayton, Md., September, 1978.) Though they are structurally similar, the respective melting points, vapor pressures at 30 degrees Celsius and decomposition temperatures of these compounds are inconsistent with what would be expected of homologs, as illustrated by Table I below:
TABLE I ______________________________________ PROPERTY TRIETHYLINDIUM TRIMETHYLINDIUM ______________________________________ Melting Point -32.degree. C. 88.degree. C. Vapor Pressure 0.8 torrs 7.2 torrs at 30.degree. C. Temperature of 40.degree. C. 150.degree. C. Onset of Decomposition ______________________________________
Trimethylindium is known to characteristically form a tetramer in the solid form and triethylindium is believed to characteristically form a loose liquid polymer structure at room temperature. This difference is believed to underlie their inconsistent properties.
The preceding table illustrates that trimethylindium is a solid at temperatures employed in bubblers. Trimethylindium has been vaporized by providing two bubblers in series to better control the amount of entrained vapor. The apparatus necessary for this two bubbler procedure is more expensive and complex, and yet provides less control of the partial pressure of trimethylindium, than apparatus used to vaporize a liquid from a single bubbler. Triethylindium has an even lower vapor pressure at 30 degrees Celsius than trimethylindium, and is also less thermally and chemically stable than trimethylindium. Triethylindium starts to decompose to indium at 40 degrees Celsius, and at an even lower temperature in the presence of hydrogen--the typical carrier gas. The vaporization of triethylindium thus must take place at a temperature approaching its decomposition temperature, and even then the deposition rate is undesirably low. The lack of homology in these indium compounds and the small number of known indium compounds have prevented those of ordinary skill in the art from selecting an optimal compound for indium MOCVD.